What is a First Flush? Is there one perfect type of First Flush device? Where do I put a First Flush on my rainwater catchment system?

First Flush or Rain Diverters, as the name implies, flush off the first water of a storm before it enters the storage tank. This is the water that could be the most contaminated by particulates, bird droppings, and other materials laying on the roof. Eliminating these contaminants before they enter into your storage and conveyance system is critical to keeping rainwater clean.

After screening your gutters, a First Flush device is the next line of defense in keeping your system and water clean. This is especially true if the water is used inside the house or where children or those with weakened immune systems may come in contact with the water.

Over the years, numerous articles have been written about First Flush devices and it is still an area of much research. However, it is generally agreed that these devices can improve the quality of the rainwater, lengthen the life of your system components, and reduce overall maintenance. The simple diagram above depicts the theory behind these devices (i.e. draining off first rainfall before it enters the tank).

Below is a summary of some the major articles and resources addressing First Flush devices. These should provide a general reference in evaluating the need for a first flush and the type of device best suited for your unique situation.

This article discusses the wide array of designs and sizes of First Flush devices with pictures of the different designs. According to this article, First Flush devices are a critical component of rainwater harvesting systems. By preventing the first, most contaminated water from the roof they:

Improve rainwater quality and safety

Help extend the life of pumps (used to distribute rainwater for use)and internal appliances (such as hot water systems, washing machines, dishwashers, etc)

Reduce tank maintenance

This article sites a study done in Australia which preliminarily concludes that the quality of the water improves dramatically with the use of First Flush water diverters.

First Flush Device Comparisons - An article by Kenneth Vidacovich published
February 2004, and available on Green Pages compares Constant-Volume Containers and First Flush Devices. This article reviews the effects of rain intensity, the general variability of rainfall intensity and maintenance issues in terms of functionality and possible health concerns. Cost estimates are made in order to compare the costs of two popularly used configurations.

This article provides an excellent overview article with detailed comparison between the two different types of devices with good instructions on how to construct a device. The author of the article is the President of FloTrue, a manufacturer of a First Flush device.

A major point from the article: "Rainfall events "generally" start at a low intensity (i.e. mist or drizzle), and rain intensity is the critical washoff factor; it is likely that a constant-volume container will substantially fill with clean water. Then, when more intense rains begin, debris and contaminates from the roof can undesirably wash into the water storage vessel. This is the major pitfall of constant-volume First Flush devices. First Flush valves can be designed to substantially overcome this pitfall."

Included in this manual is a chapter on First Flush devices and instructions on how to build a simple low-cost device.

People for Rainwater FAQ from Japan states that according to research done in March - April 1980 on rainwater collected from the roof of an office building in central Tokyo, about 0.19 - 0.39 of an inch (.5 - 1 cm) of rainfall was needed to wash the surface of the roof. Other research, done in August - September 1986, also in central Tokyo, measuring the degree of change in water quality in rainwater collected from a roof after a period of about 11 days of no rain, showed that water quality is generally stable after 0.59 - 0.78 of an inch (1.5 - 2 cm) of rain. In other words, even after a dry spell, only about the first 0.59 of an inch (1.5 cm) of rainwater is affected by pollutants accumulated on the rain-collecting surface.

Effects of First Flush on Rainwater Qualitypublished in May 2006, by the International Rainwater Catchment found that First Flush systems contribute signifcantly to reducing the contaminents that are washed off the roof harvesting area.

According to this article, rainwater users can reduce their risks of disease from contaminated rainwater consumption by regular maintenance and using a well-designed system. A range of pathogens has been found in roof-collected rainwater including Salmonella, Campylobacter, Giardia and Cryptosporidium. The likely sources of these pathogens were faecal material deposited by birds, frogs, rodents and possums, and dead animals and insects, either in the gutters or in the water tank itself.

However, detailed event studies undertaken in South East Queensland (SEQ) in Australia have shown that discarding the first .039 of an inch (~1 cm) of runoff with a First Flush device usually does little to substantially reduce the event mean concentration of bacteria and heavy metals entering a rainwater tank during a storm event.

The article also discusses a study by SEQ on first flush devices that highlighted the complexity of roof runoff chemistry. A current research project is underway which is investigating the effect of geographic location (i.e. cities, suburbs, air quality) on bulk contaminant deposition on roofs, the interaction of roofing material and rainfall characteristics on contaminant concentration in the runoff, and the transformation of contaminants that occur in a rainwater tank due to biogeochemical processes.

Rainwater Harvesting Technology Filters and Separatorsby Development Technology Unit of
the School of Engineering,
University of Warwick, covers all the different types of first flush options including: manual, fixed volume, fixed mass and flow rate. Additionally, the article includes the pros and cons of different placements (i.e. at downpipe, in downpipe, etc) of the filter options.

Key highlights from this article: 1) users will usually only spend 5-10% of cost of system on filtering the water, 2) manual systems are usually employed only in very low cost situations, and 3) filtration systems require maintenance and are usually disabled if not easy to maintain.

In summary, First Flush devices can improve the quality of your rainwater, but how much is still under debate. However, all devices that remove impurities and particulates before your water is stored are a good thing. Consequently, regardless of the proven and agreed upon health consequences, cleaning your rainwater will reduce your maintenance and lengthen the life of the system components.

Some general rules of thumb:

As a general guideline First Flush devices should remove about 10 gallons (37.8 liters) of water per 1,000 square feet (92.9 square meters) of roof/catchment area

Remember to size for each downspout of the catchment/roof area

If harvesting rainwater in an area with high pollution (e.g. urban area with poor air quality), an area with lots of dust or other pollution sources, a larger First Flush is required

First flush devices need to have an easy way to open and maintain. If not, the First Flush will most likely be disabled.